1. Field of the Invention
This invention relates to a ceramic testing method, and more particularly to a method of testing ceramics containing tetragonal zirconia.
2. Description of the Prior Art
Ceramics containing tetragonal zirconia has been extensively studied as a material for high-temperature structures, cutting tools, and oxygen sensors, because of their high heat resistivity, high mechanical strength, and high toughness and its special properties of ionic conductivity at high temperatures.
Zirconia is stable in tetragonal crystal phase at high temperatures, while it is stable in monoclinic crystal phase at low temperatures, so that when it is subjected to a temperature change in excess of a certain range, phase transformation is caused in zirconia accompanied with a considerable change in its volume.
As well known to those skilled in the art, in producing ceramics containing zirconia, it is usually fired at a high temperatures, so that zirconia contained in the ceramics at high temperatures is in tetragonal phase. In the process of being cooled, the tetragonal phase of zirconia is transformed into monoclinic phase, and such transformation is accompanied with volumetric expansion which tends to form cracks therein and deteriorate the strength thereof.
To avoid this difficulty, it has been practiced to add oxides such as yttria and magnesia into zirconia or to control the microstructure of the sintered body, so as to maintain the tetragonal zirconia, which is stable at high temperatures, as a quasi-stable phase at low temperatures. Whereby, the transformation from tetragonal phase to monoclinic phase of crystal system is minimized, and the cracks due to such transformation are eliminated, so that strong ceramics containing zirconia can be produced. However, such quasi-stable tetragonal zirconia has a shortcoming in that, although the zirconia contained in the thus produced ceramics is prevented from being transformed into its monoclinic phase for a comparatively short period of time, it is gradually transformed as time elapses, and deterioration such as occurrence of cracks and reduction in mechanical strength may result. Efforts have been made to eliminate materials which are liable to such deterioration.
With conventional ceramic testing methods, it has been very difficult to foresee possible deterioration of materials. According to a conventional practice, the subject ceramics or materials are exposed to specific use conditions for a specific period of time, and then they are tested to determine the occurrence of deterioration. Such conventional test method, which is a kind of durability test, has a shortcoming in that it is costly and it requires long test time. Thus, with the conventional ceramic testing methods, development of highly reliable ceramics containing zirconia has been difficult due to the lack of quickly obtainable test result, and users have to do without guarantee of the reliability of such ceramic products.